Process for production of terephthalic acid

ABSTRACT

The invention provides a method for producing terephthalic acid, characterized by including subjecting a p-phenylene compound to liquid-phase oxidation reaction by use of a molecular-oxygen-containing gas in the presence of a catalyst at least containing a heavy metal compound and a bromine compound, and hydrous acetic acid having a water content of 1 to 15 mass %, to thereby yield a slurry; regulating the temperature of the slurry to 35 to 140° C., to thereby cause terephthalic acid to precipitate; removing the terephthalic acid through solid-liquid separation, to thereby recover a mother liquor; and recovering the catalyst from the mother liquor through a series of the following steps (1) to (4) for reusing at least a portion of the catalyst in the liquid-phase oxidation reaction: (1) an adsorption step including regulating the ratio “amount by mole of bromide ions in the mother liquor/total amount by mole of heavy metal ions in the mother liquor” to 0.6 to 3, and then exposing the mother liquor to a pyridine-ring-containing chelate resin which has been heated to 35 to 140° C., so that the resin adsorbs catalyst-derived heavy metal ions and bromide ions, and also adsorbs a carboxylic acid mixture which has been by-produced through the liquid-phase oxidation reaction, (2) an elution step (A) of exposing hydrous acetic acid having a water content of 1 to 15 mass % to the pyridine-ring-containing chelate resin which has undergone the adsorption step, thereby yielding an eluate containing the by-produced carboxylic acid mixture, (3) an elution step (B) of exposing water or hydrous acetic acid having a water content of 20 mass % or more to the pyridine-ring-containing chelate resin which has undergone the elution step (A), thereby yielding an eluate containing catalyst-derived heavy metal ions and bromide ions, and (4) a displacement step of exposing hydrous acetic acid having a water content of 1 to 15 mass % to the pyridine-ring-containing chelate resin which has undergone the elution step (B), serving as a displacement liquid, thereby regenerating the resin.

TECHNICAL FIELD

The present invention relates to a method for producing terephthalicacid. Terephthalic acid is effectively employed as, for example, astarting material for polyethylene terephthalate.

BACKGROUND ART

Generally, terephthalic acid is produced through liquid-phase oxidationreaction of a p-phenylene compound (e.g., p-xylene) in acetic acidserving as a solvent in the presence of a catalyst (e.g., cobalt ormanganese), or in the presence of a catalyst together with a promoter(e.g., a bromine compound or acetaldehyde). A slurry containing crudeterephthalic acid produced through such liquid-phase oxidation reactionis generally subjected to crystallization at ambient pressure andlowered temperature, followed by solid-liquid separation.

A mother liquor recovered through the solid-liquid separation containscatalyst-derived useful catalyst components such as heavy metal ions andbromide ions, and an industrial process requires recycling of thesecatalyst components for reduction of production cost. In the mostconvenient recycling method, the mother liquor is returned, as it is, toand reused in the reaction system. However, as has been known, since themother liquor contains, for example, various organic impuritiesby-produced through liquid-phase oxidation reaction, and inorganicimpurities derived from corrosion of an employed apparatus, when themother liquor is reused as it is in the reaction system, theconcentration of these impurities is gradually increased in the reactionsystem, and an increase in impurity concentration beyond a predeterminedlevel adversely affects liquid-phase oxidation reaction. For example, inthe case of production of terephthalic acid, the mother liquor isgenerally returned to the reaction system in a proportion of 70 to 98%,and the remaining portion (2 to 30%) of the mother liquor (i.e., aportion of the mother liquor which is not reused in the reaction system)is fed to a step of recovering acetic acid serving as a solvent. In viewof the foregoing, various methods have been proposed for recoveringcatalyst components from the mother liquor fed to such an acetic acidrecovery step, and reusing the catalyst components.

For example, such known methods include a method in which water and analkali metal carbonate salt are added to a residue obtained throughrecovery of a solvent from a mother liquor, to thereby cause a catalystcomponent in the form of carbonate salt to precipitate, and the catalystcomponent is dissolved in a predetermined amount of acetic acid servingas a solvent, and reused for reaction (see Patent Document 1); and amethod in which oxalic acid and an alkali metal hydroxide are added to amother liquor, to thereby cause a catalyst component in the form ofoxalate salt to precipitate, and the catalyst component is dissolved inacetic acid serving as a solvent, followed by oxidation for recovery ofthe catalyst component (see Patent Document 2). There have also beenknown methods for recovering a catalyst component from a mother liquorby use of an anion exchange resin (see Patent Documents 3 to 11),including a method in which a bromide-ion-type anion exchange resin isexposed to a mother liquor for adsorption of cobalt ions and manganeseions on the resin, hydrous acetic acid having a water content of 2 mass% and water are caused to pass through the resin, to thereby recovercobalt ions and manganese ions through elution, alower-aliphatic-monocarboxylate-ion-type weakly basic anion exchangeresin is exposed to an eluate obtained through the aforementionedadsorption for adsorption of bromide ions and nickel ions on the resin,and hydrous acetic acid having a water content of 2 mass % and water arecaused to pass through the resin, to thereby recover bromide ions andnickel ions through elution (see Patent Document 3); a method in whichthe cobalt concentration and the bromine/cobalt ratio of a mother liquorare respectively regulated so as to fall within specific ranges,followed by adsorption of cobalt and bromine on a strongly basic anionexchange resin, and cobalt and bromine are eluted from the stronglybasic anion exchange resin with hydrous acetic acid having a watercontent of 10 mass % or more, to thereby recover a cobalt catalyst (seePatent Document 4); a method in which cobalt ions, manganese ions, andbromide ions are caused to be adsorbed together on an anion exchangeresin containing a pyridine ring serving as an ion exchange group, andthese ions are recovered through elution by a known technique (seePatent Document 5); and a method in which a catalyst component isrecovered from a mother liquor by use of an anion-exchange-type chelateresin (see Patent Document 7).

-   Patent Document 1: Japanese Patent Application Laid-Open (kokai) No.    S48-066090-   Patent Document 2: Japanese Patent Application Laid-Open (kokai) No.    H02-203939-   Patent Document 3: Japanese Patent Application Laid-Open (kokai) No.    S53-104590-   Patent Document 4: Japanese Patent Application Laid-Open (kokai) No.    S53-133574-   Patent Document 5: Japanese Patent Application Laid-Open (kokai) No.    S53-102290-   Patent Document 6: Japanese Patent Application Laid-Open (kokai) No.    H10-015390-   Patent Document 7: Japanese Patent Application Laid-Open (kokai) No.    H11-152246-   Patent Document 8: Japanese Patent Application Laid-Open (kokai) No.    2002-012573-   Patent Document 9: Specification of US Patent No. 4162991-   Patent Document 10: Specification of US Patent No. 4238294-   Patent Document 11: Japanese Kohyo Patent Publication No.    2003-507160

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The method described in Patent Document 1 or 2 requires, for example, analkali metal carbonate salt or oxalic acid in an amount equivalent to orgreater than that of a catalytic metal, and thus is economicallydisadvantageous. In addition, the method requires an intricate processand a great deal of labor for recovery of a catalyst component in theform of carbonate salt or oxalate salt, which is not preferred.

The anion exchange resins described in Patent Documents 3 to 6 and 8 to11 exhibit low heat resistance (allowable temperature limit: 80 to 100°C.), and thus may undergo chemical degradation (reduction in exchangecapacity) or physical degradation (e.g., breakage of the resin)depending on the temperature of a mother liquor which has been caused topass through the resins, which is not preferred. When apyridine-ring-containing anion exchange resin described in PatentDocument 5 is employed, elution of cobalt ions, manganese ions, andbromide ions adsorbed on the resin requires a liquid containing, forexample, sulfuric acid, nitric acid, alkylsulfuric acid, or hydroxideions, and, in practice, difficulty is encountered in reusing thethus-recovered liquid as is in the reaction system.

The method described in Patent Document 7 employs an anion-exchange-type“chelate resin,” and is advantageous in that, unlike the case of thepyridine-ring-containing anion exchange resin described in PatentDocument 5, cobalt ions, manganese ions, and bromide ions adsorbed onthe chelate resin can be eluted with hydrous acetic acid having a watercontent of 1 to 15 mass %. However, when a mother liquor recoveredthrough the method of the present invention (i.e., a mother liquorrecovered through solid-liquid separation of a slurry produced byliquid-phase oxidation reaction of a p-phenylene compound) is caused topass through the chelate resin, a carboxylic acid mixture which has beenby-produced (hereinafter may be referred to as a “by-produced carboxylicacid mixture”) is adsorbed on the chelate resin, and, upon elution ofcobalt ions, manganese ions, and bromide ions, the by-producedcarboxylic acid mixture is eluted and recovered together with theseions. When the by-produced carboxylic acid mixture concentration of themother liquor is increased, a catalyst is deactivated, and reactionyield is reduced. Therefore, a catalyst recovery method employing theanion-exchange-type chelate resin has not yet been put into practice onan industrial scale.

In view of the foregoing, an object of the present invention for solvingthe aforementioned problems is to provide a method for producing, in anindustrially advantageous manner, terephthalic acid by use of a specificchelate resin while suppressing an increase in concentration of aby-produced carboxylic acid mixture in a reaction system and a motherliquor.

Means for Solving the Problems

In order to attain the aforementioned object, the present inventors haveconducted extensive studies, and as a result have found that, in theproduction of terephthalic acid, when catalyst components are recoveredfrom a mother liquor by use of an anion-exchange-typepyridine-ring-containing chelate resin through a series of thebelow-described steps (1) to (4), heavy metal ions and bromide ions(i.e., catalyst components) can be separated from a by-producedcarboxylic acid mixture, and, even in the case where liquid-phaseoxidation reaction is carried out continuously, an increase inconcentration of the by-produced carboxylic acid mixture can besuppressed in the reaction system and the mother liquor, and thereforethe catalyst components can be recovered and reused a maximum number oftimes in liquid-phase oxidation reaction without being deactivated. Thepresent invention has been accomplished on the basis of this finding.

Accordingly, the present invention provides:

-   [1] a method for producing terephthalic acid, characterized by    comprising:

subjecting a p-phenylene compound to liquid-phase oxidation reaction byuse of a molecular-oxygen-containing gas in the presence of a catalystat least containing a heavy metal compound and a bromine compound, andhydrous acetic acid having a water content of 1 to 15 mass %, to therebyyield a slurry;

regulating the temperature of the slurry to 35 to 140° C., to therebycause terephthalic acid to precipitate;

removing the terephthalic acid through solid-liquid separation, tothereby recover a mother liquor; and

recovering the catalyst from the mother liquor through a series of thefollowing steps (1) to (4) for reusing at least a portion of thecatalyst in the liquid-phase oxidation reaction:

(1) an adsorption step including regulating the ratio “amount by mole ofbromide ions in the mother liquor/total amount by mole of heavy metalions in the mother liquor” to 0.6 to 3, and then exposing the motherliquor to a pyridine-ring-containing chelate resin which has been heatedto 35 to 140° C., so that the resin adsorbs catalyst-derived heavy metalions and bromide ions, and also adsorbs a carboxylic acid mixture whichhas been by-produced through the liquid-phase oxidation reaction(hereinafter the carboxylic acid mixture will be referred to as a“by-produced carboxylic acid mixture”),

(2) an elution step (A) of exposing hydrous acetic acid having a watercontent of 1 to 15 mass % to the pyridine-ring-containing chelate resinwhich has undergone the adsorption step, thereby yielding an eluatecontaining the by-produced carboxylic acid mixture,

(3) an elution step (B) of exposing water or hydrous acetic acid havinga water content of 20 mass % or more to the pyridine-ring-containingchelate resin which has undergone the elution step (A), thereby yieldingan eluate containing catalyst-derived heavy metal ions and bromide ions,and

(4) a displacement step of exposing hydrous acetic acid having a watercontent of 1 to 15 mass % to the pyridine-ring-containing chelate resinwhich has undergone the elution step (B), serving as a displacementliquid, thereby regenerating the resin;

-   [2] the method for producing terephthalic acid as described in [1]    above, wherein hydrous acetic acid is recovered from the mother    liquor which has undergone the adsorption step, from the eluate    obtained through the elution step (A), and from the displacement    liquid employed in the displacement step, and the recovered hydrous    acetic acid is reused in the liquid-phase oxidation reaction as at    least a portion of hydrous acetic acid having a water content of 1    to 15 mass %;-   [3] the method for producing terephthalic acid as described in [1]    or [2] above, wherein hydrous acetic acid is recovered from the    mother liquor which has undergone the adsorption step, from the    eluate obtained through the elution step (A), and from the    displacement liquid employed in the displacement step, and the    recovered hydrous acetic acid is reused in the elution step (A) as    at least a portion of hydrous acetic acid having a water content of    1 to 15 mass %;-   [4] the method for producing terephthalic acid as described in any    one of [1] to [3] above, wherein hydrous acetic acid is recovered    from the mother liquor which has undergone the adsorption step, from    the eluate obtained through the elution step (A), and from the    displacement liquid employed in the displacement step, and the    recovered hydrous acetic acid is reused in the displacement step as    a displacement liquid;-   [5] the method for producing terephthalic acid as described in any    one of [1] to [4] above, wherein the eluate obtained through the    elution step (B) is returned to the liquid-phase oxidation reaction,    and reused as at least a portion of the catalyst;-   [6] the method for producing terephthalic acid as described in any    one of [1] to [5] above, wherein a regenerated    pyridine-ring-containing chelate resin which has been obtained    through the displacement step is reused in the adsorption step as    the pyridine-ring-containing chelate resin;-   [7] the method for producing terephthalic acid as described in any    one of [1] to [6] above, wherein the hydrous acetic acid having a    water content of 1 to 15 mass % and employed in the elution step (A)    contains bromide ions in an amount of 1 to 1,000 mass ppm;-   [8] the method for producing terephthalic acid as described in any    one of [1] to [7] above, wherein the hydrous acetic acid having a    water content of 1 to 15 mass % and employed in the displacement    step as a displacement liquid contains bromide ions in an amount of    1 to 1,000 mass ppm;-   [9] the method for producing terephthalic acid as described in any    one of [1] to [8] above, wherein, in the adsorption step, the ratio    “amount by mole of bromide ions in the mother liquor/total amount by    mole of heavy metal ions in the mother liquor” is regulated to 1.6    to 2.5; and-   [10] the method for producing terephthalic acid as described in any    one of [1] to [9] above, wherein when Q represents the total    amount (g) of hydrous acetic acid having a water content of 1 to 15    mass % supplied in the elution step (A), and V represents the volume    (mL) of a pyridine-ring-containing chelate resin layer, the ratio    Q/V is 0.5 to 10.

EFFECTS OF THE INVENTION

In the method for producing of terephthalic acid, a by-producedcarboxylic acid mixture can be effectively separated fromcatalyst-derived heavy metal ions and bromide ions, and thethus-recovered heavy metal ions and bromide ions can be reused, as itis, as a catalyst in liquid-phase oxidation reaction. Therefore,terephthalic acid can be produced stably in a more economicallyadvantageous manner while high reaction yield is maintained over a longperiod of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary process for producing terephthalic acid.

DESCRIPTION OF REFERENCE NUMERALS

-   1: Reactor-   2: Crystallization step-   3: Solid-liquid separator-   4: Mother liquor receiving tank-   5: Pyridine-ring-containing chelate resin column-   6: Hydrobromic acid tank-   7: Tank for storing hydrous acetic acid having a water content of    about 12 mass % (also called “displacement liquid tank”)-   8: Tank for storing hydrous acetic acid having a water content of    about 35 mass %-   9: Intermediate tank-   10: Raw material tank-   11: Conduit for cake of crude terephthalic acid crystals-   12: Conduit for mother liquor-   13: Conduit for hydrobromic acid-   14: Conduit for prepared mother liquor-   15: Conduit over column-   16: Conduit under column-   17: Conduit for purged liquid-   18: Conduit for hydrous acetic acid having a water content of about    12 mass % (also called “displacement liquid conduit”)-   19: Conduit for hydrous acetic acid having a water content of about    35 mass %-   20: Conduit for recovered catalyst liquid-   21: Conduit for recovered catalyst liquid-   22: Conduit for raw material mixture

BEST MODES FOR CARRYING OUT THE INVENTION

In the present invention, firstly, a p-phenylene compound is subjectedto liquid-phase oxidation reaction by use of amolecular-oxygen-containing gas in the presence of a catalyst at leastcontaining a heavy metal compound and a bromine compound, and hydrousacetic acid having a water content of 1 to 15 mass % (preferably 1 to 12mass %, more preferably 1 to 9 mass %), to thereby yield a slurry(hereinafter the thus-obtained slurry may be referred to as an“oxidation reaction slurry”). As used herein, the term “p-phenylenecompound” refers to a phenylene compound having substituents atpositions 1 and 4 of the benzene ring. Examples of such a substituentinclude alkyl groups such as methyl, ethyl, propyl, and butyl. Aparticularly preferred p-phenylene compound is p-xylene.

At least one of a cobalt compound and a manganese compound isessentially employed as a heavy metal compound (i.e., a catalystcomponent), and optionally, for example, a nickel compound, a ceriumcompound, or a zirconium compound may be employed together with such anessential heavy metal compound. Examples of the cobalt compound,manganese compound, nickel compound, cerium compound, and zirconiumcompound include organic acid salts, hydroxides, halides, and carbonatesof the respective metals. Particularly, acetic acid salts and bromidesof the respective metals are preferably employed. Specific examples ofsuch a heavy metal compound include cobalt acetate, cobalt hydroxide,cobalt fluoride, cobalt chloride, cobalt bromide, cobalt iodide, cobaltcarbonate, manganese acetate, manganese hydroxide, manganese fluoride,manganese chloride, manganese bromide, manganese iodide, manganesecarbonate, nickel acetate, nickel hydroxide, nickel fluoride, nickelchloride, nickel bromide, nickel iodide, nickel carbonate, ceriumacetate, cerium hydroxide, cerium fluoride, cerium chloride, ceriumbromide, cerium iodide, cerium carbonate, zirconium acetate, zirconiumhydroxide, zirconium fluoride, zirconium chloride, zirconium bromide,zirconium iodide, and zirconium carbonate.

No particular limitation is imposed on the bromine compound employed asa catalyst component, so long as it dissolves in hydrous acetic acid andgenerates bromide ions in the reaction system. Examples of the brominecompound include inorganic bromine compounds such as hydrogen bromide,sodium bromide, and cobalt bromide; and organic bromine compounds suchas bromoacetic acid and tetrabromoethane. Of these, hydrogen bromide,cobalt bromide, and manganese bromide are preferably employed. Morepreferably, hydrobromic acid prepared by dissolving hydrogen bromide inan aqueous solution is employed.

The liquid-phase oxidation reaction is preferably performed at 160 to230° C., more preferably 180 to 210° C. When the reaction temperaturefalls within the above range, the amount of a reaction intermediateremaining in the oxidation reaction slurry can be reduced, and excessiveloss of hydrous acetic acid having a water content of 1 to 15 mass %(i.e., a solvent) by combustion is prevented. No particular limitationis imposed on the reaction pressure, so long as the reaction system canbe maintained in a liquid phase at the aforementioned reactiontemperature. Generally, the reaction pressure is preferably 0.78 to 3.04MPa, more preferably 0.98 to 1.86 MPa.

Examples of a molecular-oxygen-containing gas include air, oxygen gasdiluted with an inert gas, and oxygen-enriched air. Generally, air ispreferred, from the viewpoints of facility and cost.

Preferably, the oxidation reaction slurry obtained through theaforementioned liquid-phase oxidation reaction is fed to the subsequentreactor connected in series, so as to thoroughly complete oxidationreaction by use of a molecular-oxygen-containing gas.

The thus-obtained oxidation reaction slurry is subjected to pressurereduction and cooling (35 to 140° C.) through flash evaporation in oneor more stages in a crystallization step employing one or morecrystallization tanks, to thereby thoroughly crystallize crudeterephthalic acid, and then the slurry is fed to the below-describedsolid-liquid separator.

The oxidation reaction slurry obtained through the aforementionedliquid-phase oxidation reaction is separated into crude terephthalicacid crystals and a mother liquor by means of a solid-liquid separator.This solid-liquid separation is generally performed at atmosphericpressure. No particular limitation is imposed on the separationtemperature, but this separation is generally performed at a temperaturelower than the boiling point of hydrous acetic acid at atmosphericpressure (e.g., 35 to 110° C). Examples of the solid-liquid separatorinclude a centrifugal separator, a centrifugal filter, and a vacuumfilter. This solid-liquid separation is preferably performed so that thecrystal content of the mother liquor is 1 mass % or less, morepreferably 0.1 mass % or less.

The thus-obtained crude terephthalic acid crystals may be appropriatelysubjected to a known purification process, such as catalytichydrogenation (see, for example, Japanese Patent Publication (kokoku)No. S41-16860), oxidation treatment, or recrystallization, to therebyyield terephthalic acid crystals of high purity.

<Adsorption Step>

The mother liquor recovered through separation of crude terephthalicacid crystals from the oxidation reaction slurry by the aforementionedprocedure contains catalyst-derived heavy metal ions and bromide ions,and a by-produced carboxylic acid mixture. These heavy metal ions areions of the heavy metals constituting the aforementioned heavy metalcompounds. The by-produced carboxylic acid mixture includes aromaticpolycarboxylic acids each having carboxyl groups which are ortho to eachother. Examples of such an aromatic polycarboxylic acid includetrimellitic acid compounds (e.g., trimellitic acid, hemimellitic acid,and 5-methyltrimellitic acid); pyromellitic acid; and phthalic acidcompounds (e.g., phthalic acid, 3-methylphthalic acid, and4-methylphthalic acid). The mother liquor contains, in addition to theaforementioned ions and by-produced carboxylic acid mixture, variousorganic compounds (e.g., terephthalic acid, acetic acid, unreacted rawmaterials, reaction intermediates, and reaction by-products), and aconsiderable amount of water (i.e., a reaction product).

In the present invention, the ratio “amount by mole of bromide ions inthe mother liquor/total amount by mole of heavy metal ions in the motherliquor” (hereinafter the ratio may be referred to as “the bromide ratio(of the mother liquor)”) is regulated, and then apyridine-ring-containing chelate resin is exposed to the mother liquor,to thereby selectively adsorb catalyst-derived heavy metal ions, bromideions, and a by-produced carboxylic acid mixture on thepyridine-ring-containing chelate resin, whereby a liquid (hereinafterthe thus-obtained liquid may be referred to as a “residual motherliquor”) is recovered [adsorption step]. When crude terephthalic acidcrystals recovered through separation by means of a solid-liquidseparator are washed with water or hydrous acetic acid, a liquidobtained through this washing may be mixed with the mother liquor, andthe resultant mixture may be subjected to the adsorption step.

The pyridine-ring-containing chelate resin employed in the presentinvention is an anion-exchange-type chelate resin having a pyridine ringand obtained through polymerization of 4-vinylpyridine anddivinylbenzene serving as main raw materials. In general, a chelateresin is a water-insoluble polymer base having a ligand which cancoordinate with metal ions to form a complex, and exhibits a function ofselectively adsorbing/separating specific metal ions. Particularly, a‘pyridine-ring-containing’ chelate resin is advantageous in that iteffectively adsorbs heavy metal ions. Such a pyridine-ring-containingchelate resin may be a commercially available one. Examples ofcommercially available pyridine-ring-containing chelate resins include“REILLEX (registered trademark) 425 Polymer” (trade name, product ofReilly) and “Sumichelate (registered trademark) CR-2” (trade name,product of Sumitomo Chemical Co., Ltd.).

No particular limitation is imposed on the method for exposing themother liquor to a pyridine-ring-containing chelate resin, and, forexample, the chelate resin is impregnated with the mother liquor, or themother liquor is caused to pass through the chelate resin. From theviewpoint of adsorption efficiency, more preferably, the mother liquoris caused to pass through a pyridine-ring-containing chelate resin. Fromthe viewpoints of heat resistance and adsorption performance of apyridine-ring-containing chelate resin, the chelate resin must be heatedto 35 to 140° C. before being exposed to the mother liquor. The chelateresin is preferably heated to 45 to 130° C., more preferably 70 to 120°C., much more preferably 85 to 110° C.

The residual mother liquor obtained by exposing the mother liquor to apyridine-ring-containing chelate resin contains hydrous acetic acidhaving a water content of 1 to 15 mass %. Therefore, preferably, thehydrous acetic acid is recovered from the residual liquid through, forexample, distillation, and at least a portion of the thus-recoveredhydrous acetic acid is reused in liquid-phase oxidation reaction, oremployed as a displacement liquid for the below-described regenerationof the pyridine-ring-containing chelate resin.

In the present invention, the mother liquor obtained throughliquid-phase oxidation reaction and solid-liquid separation generallyhas a water content of 7 to 16 mass %. Therefore, when the mother liquoris applied to a pyridine-ring-containing chelate resin column, in manycases, the water content of the mother liquor is not particularlyrequired to be regulated. However, when the water content of the motherliquor is increased by, for example, mixing the mother liquor with aliquid obtained through the aforementioned washing of crude terephthalicacid crystals recovered by solid-liquid separation, the water content ispreferably regulated to 16 mass % or less (more preferably 1 to 14 mass%, much more preferably 5 to 12 mass %) through, for example,distillation. When the water content of the mother liquor exceeds 16mass %, in the adsorption step, catalyst-derived heavy metal ions andbromide ions and a by-produced carboxylic acid mixture are less likelyto be sufficiently adsorbed on a pyridine-ring-containing chelate resin.

The pyridine-ring-containing chelate resin which has not undergone anypreliminary treatment may be exposed to the mother liquor. However,preferably, the mother liquor is brought into contact with abromide-type chelate resin prepared by exposing apyridine-ring-containing chelate resin to, for example, an acetic acidsolution containing bromide ions in advance. No particular limitation isimposed on the method for preparing a bromide-typepyridine-ring-containing chelate resin, and, for example, the chelateresin may be prepared by exposing a pyridine-ring-containing chelateresin to an aqueous solution of any of the aforementioned brominecompounds (e.g., sodium bromide and hydrogen bromide) or a liquidmixture of the aqueous solution and acetic acid, followed by removal ofexcess bromide through washing with glacial acetic acid or hydrousacetic acid having a water content of 15 mass % or less. This washing ispreferably performed with hydrous acetic acid having a water contentlower than that of the mother liquor.

From the viewpoint of efficient elution of a by-produced carboxylic acidmixture adsorbed on a pyridine-ring-containing chelate resin, thebromide ratio of the mother liquor is 0.6 to 3, preferably 0.7 to 2.8,more preferably 0.9 to 2.5, much more preferably 1.6 to 2.5,particularly preferably 2 to 2.5. When the bromide ratio is high,percent adsorption of the aforementioned heavy metal ions is increased,and percent adsorption of a by-produced carboxylic acid mixture tends tobe reduced. Therefore, when the bromide ratio is increased particularlyin the adsorption step, in the below-described elution step (A), aby-produced carboxylic acid mixture can be effectively separated fromcatalyst components (i.e., heavy metal ions and bromide ions). Thebromide ratio is regulated by, for example, adding, to the motherliquor, an aqueous solution of any of the aforementioned brominecompounds (e.g., hydrobromic acid) serving as a bromide source.

<Elution Steps (A) and (B)>

In the present invention, the pyridine-ring-containing chelate resinwhich has undergone the aforementioned adsorption step is subjected toan elution step (A); i.e., a step of exposing the resin to hydrousacetic acid having a water content of 1 to 15 mass % (preferably 1 to 12mass %, more preferably 1 to 9 mass %), thereby selectively eluting aby-produced carboxylic acid mixture, and then is subjected to an elutionstep (B); i.e., a step of exposing the resin to water or hydrous aceticacid having a water content of 20 mass % or more, thereby recoveringcatalyst-derived heavy metal ions and bromide ions.

The pyridine-ring-containing chelate resin which has undergone theaforementioned adsorption step contains, in addition to catalyst-derivedheavy metal ions and bromide ions, a by-produced carboxylic acidmixture. When the pyridine-ring-containing chelate resin is subjected tothe elution step (B) without being subjected to the elution step (A), soas to recover the heavy metal ions and bromide ions (i.e., catalystcomponents), the by-produced carboxylic acid mixture is contained in thecatalyst components. In such a case, when the catalyst components arereturned to and reused in the reaction system, the concentration of theby-produced carboxylic acid mixture is gradually increased in thereaction system and the mother liquor, which causes deactivation of thecatalyst. Such a process results in an economic disadvantage when beingput into practice on an industrial scale. In order to suppress anincrease in concentration of the by-produced carboxylic acid mixture,the elution step (A) is required; i.e., the by-produced carboxylic acidmixture is selectively eluted, with heavy metal ions and bromide ionsbeing adsorbed on the pyridine-ring-containing chelate resin.

The hydrous acetic acid having a water content of 1 to 15 mass % andemployed in the elution step (A) preferably contains bromide ions in anamount of 1 to 1,000 mass ppm, more preferably 10 to 1,000 mass ppm, forrapidly eluting the by-produced carboxylic acid mixture adsorbed on thepyridine-ring-containing chelate resin. In the elution step (A), theremay be employed acetic acid (water content: 4 to 11 mass %, bromide ioncontent: 1 to 50 mass ppm) recovered from the bottom of a distillationcolumn upon removal of water (through evaporation) from the residualmother liquor obtained through the adsorption step, an eluate obtainedthrough the elution step (A), or a displacement liquid employed in thebelow-described displacement step.

When Q represents the total amount (g) of hydrous acetic acid suppliedin the elution step (A), and V represents the volume (mL) of apyridine-ring-containing chelate resin layer, the ratio Q/V ispreferably 0.5 to 10, more preferably 1 to 6, much more preferably 3 to4.5. When the ratio Q/V falls within the above range, the by-producedcarboxylic acid mixture can be effectively and selectively eluted fromthe pyridine-ring-containing chelate resin.

Preferably, hydrous acetic acid having a water content of 1 to 15 mass %is recovered, through distillation or a similar technique, from theeluate obtained through the elution step (A), which contains theby-produced carboxylic acid mixture, and at least a portion of thethus-recovered hydrous acetic acid is reused in liquid-phase oxidationreaction, or employed in the elution step (A) or in the below-describeddisplacement step.

In the elution step (B), metal impurities other than catalyst-derivedheavy metal ions and bromide ions are not virtually adsorbed on thepyridine-ring-containing chelate resin. Therefore, through exposing theresin to water or hydrous acetic acid having a water content of 20 mass% or more (preferably 20 to 70 mass %, more preferably 25 to 50 mass %,much more preferably 25 to 40 mass %), hydrous acetic acid whichcontains heavy metal ions and bromide ions, and which can be reused, asit is, in liquid-phase oxidation reaction (hereinafter the thus-obtainedhydrous acetic acid may be referred to as a “recovered catalyst liquid”)can be produced.

Alternatively, a condensate (water content: 20 to 50 mass %) recoveredin liquid-phase oxidation reaction by means of a reflux condenserattached to the reactor may be employed in the elution step (B) ashydrous acetic acid.

<Displacement Step>

In this step, from the viewpoint of adsorption efficiency of catalystcomponents, the pyridine-ring-containing chelate resin which hasundergone the elution step (B) is brought into contact with adisplacement liquid; i.e., hydrous acetic acid having a water content of1 to 15 mass % (preferably 1 to 12 mass %, more preferably 1 to 9 mass%), to thereby regenerate the pyridine-ring-containing chelate resin.The thus-regenerated pyridine-ring-containing chelate resin can bereused in the adsorption step. Through the displacement step, the watercontent of hydrous acetic acid present around the chelate resin isreduced to a level equal to the water content of the displacementliquid, so that heavy metal ions and bromide ions are rapidly adsorbedon the chelate resin in the subsequent adsorption step. In contrast,when the displacement step is not performed, since the chelate resinlayer is surrounded by hydrous acetic acid having a high water contentimmediately after the elution step (B), in the adsorption step,adsorption efficiency of catalyst components is reduced at an earlystage of exposure of the chelate resin to the mother liquor, and percentrecovery of the catalyst components is reduced, resulting in an economicdisadvantage.

In order to facilitate adsorption of cobalt ions, manganese ions, andbromide ions on the pyridine-ring-containing chelate resin, morepreferably, hydrous acetic acid having a water content of 1 to 15 mass %and containing bromide ions in an amount of 1 to 1,000 mass ppm isemployed as a displacement liquid.

There may be employed, as a displacement liquid, acetic acid (watercontent: 4 to 11 mass %, bromide ion content: 1 to 50 mass ppm)recovered from the bottom of a distillation column upon removal of water(through evaporation) from the residual mother liquor obtained throughthe adsorption step, the eluate obtained through the elution step (A),or the displacement liquid employed in the displacement step.

The overall flow of the method of the present invention will next bebriefly described with reference to FIG. 1, which shows an exemplaryprocess for producing terephthalic acid. However, the present inventionis not limited to the process shown in FIG. 1.

As shown in FIG. 1, a mixture of raw materials (a p-phenylene compound,a heavy metal compound, a bromine compound, and hydrous acetic acid) isfed through a conduit 22 to a reactor 1, and liquid-phase oxidationreaction is performed in the presence of a molecular-oxygen-containinggas. The reaction product is in the form of slurry obtained throughcrystallization of a portion of crude terephthalic acid. The reactionproduct is subjected to pressure reduction and cooling through flashevaporation in several stages in a crystallization step 2, followed byseparation by means of a solid-liquid separator 3, to thereby yield acake of crude terephthalic acid crystals. The thus-obtained cake istransferred through a conduit 11. The mother liquor recovered throughseparation by means of the solid-liquid separator 3 is fed via a conduit12 to a mother liquor receiving tank 4, and hydrobromic acid is fed froma hydrobromic acid tank 6 through a conduit 13 to the mother liquorreceiving tank 4, to thereby appropriately regulate the bromide ratio ofthe mother liquor. Subsequently, the mother liquor is caused to passthrough conduits 14 and 15, and then subjected to the adsorption step;i.e., the mother liquor is introduced to a pyridine-ring-containingchelate resin column 5 through the top of the column and caused to passtherethrough. A substance which is not adsorbed on thepyridine-ring-containing chelate resin is discharged from the bottom ofthe column, and then purged via conduits 16 and 17. The substance isappropriately subjected to, for example, distillation for recovery ofhydrous acetic acid, and the thus-recovered hydrous acetic acid istransferred (for reuse) to a raw material tank 10 or a tank 7 forstoring hydrous acetic acid having a water content of about 12 mass %.

The pyridine-ring-containing chelate resin which has undergone theadsorption step is subjected to the elution step (A). Specifically,hydrous acetic acid having a water content of about 12 mass % is fedfrom the tank 7 through a conduit 18 and the conduit 15, and introducedto the pyridine-ring-containing chelate resin column 5 through the topof the column and caused to pass therethrough. An eluate containing aby-produced carboxylic acid mixture is discharged from the bottom of thecolumn, and then purged via the conduits 16 and 17. The eluate isappropriately subjected to, for example, distillation for recovery ofhydrous acetic acid, and the thus-recovered hydrous acetic acid istransferred (for reuse) to the raw material tank 10 or the tank 7 forstoring hydrous acetic acid having a water content of about 12 mass %.

After completion of the elution step (A), the elution step (B) iscarried out. Specifically, hydrous acetic acid having a water content ofabout 35 mass % is fed from a tank 8 through a conduit 19 and theconduit 16, and introduced to the pyridine-ring-containing chelate resincolumn 5 so as to pass therethrough for elution of heavy metal ions andbromide ions adsorbed on the pyridine-ring-containing chelate resin. Thethus-recovered catalyst liquid is transferred through a conduit 20 to anintermediate tank 9. Subsequently, the recovered catalyst liquid is fedthrough a conduit 21 to the raw material tank 10, and reused as acatalyst for liquid-phase oxidation reaction.

After completion of the elution step (B), the displacement step iscarried out. Specifically, hydrous acetic acid having a water content ofabout 12 mass %, serving as a displacement liquid, is fed from thedisplacement liquid tank 7 through the conduit 18 and the conduit 15,and introduced to the pyridine-ring-containing chelate resin column 5 soas to pass therethrough. The resultant eluate is purged via the conduits16 and 17. The eluate is appropriately subjected to, for example,distillation for recovery of hydrous acetic acid, and the thus-recoveredhydrous acetic acid is transferred (for reuse) to the raw material tank10 or the tank 7 for storing hydrous acetic acid having a water contentof about 12 mass %. After completion of the displacement step, theaforementioned adsorption step may be carried out.

Examples

The present invention will next be described in more detail by way ofexamples, which should not be construed as limiting the inventionthereto.

<Preliminary Treatment of Pyridine-Ring-Containing Chelate Resin>

In each of the Examples and the Comparative Example, there was employeda pyridine-ring-containing chelate resin prepared through the followingprocedure: an acetic acid solution (200 mL) containing 10 mass %hydrobromic acid was caused to pass through a pyridine-ring-containingchelate resin [“Sumichelate (registered trademark) CR-2” (trade name,product of Sumitomo Chemical Co., Ltd.)] so as to prepare a bromide-typepyridine-ring-containing chelate resin, and excess hydrobromic acid wasrinsed out the chelate resin with an acetic acid solution having a watercontent of 12 mass %.

<Method for Determining Heavy Metal Ion Concentration>

Heavy metal ion concentration was determined by means of an atomicabsorption spectrophotometer having the following specification.

-   Model: Polarized Zeeman atomic absorption spectrophotometer Z-2300    (product of Hitachi High-Technologies Corporation)-   Wavelength: Cobalt ion: 240.7 nm, Manganese ion: 279.6 nm Flame:    Acetylene-air-   Determination method: An appropriate amount of a sample is placed in    a 100-mL glass container (mass of the sample is measured by means of    a balance), and the sample is diluted with pure water and 20 mass %    hydrochloric acid (constant boiling point, iron-free) for accurate    analysis (about 2 mL) so that the concentration of heavy metal ions    to be measured is about 1 ppm (the mass of the diluted sample is    measured for determination of a dilution factor). The heavy metal    ion concentration of the diluted sample is determined on the basis    of a calibration curve prepared by use of standard samples having    heavy metal ion concentrations of 0 ppm, 1 ppm, and 2 ppm. The heavy    metal ion concentration of the undiluted sample is determined by    multiplying the heavy metal ion concentration of the diluted sample    by the dilution factor.

<Method for Determining Bromide Ion Concentration>

Bromide ion concentration was determined under the following conditions.

-   Titrator: Automatic potentiometric titrator AT-510 (product of Kyoto    Electronics Manufacturing Co., Ltd.)-   Titration liquid: 1/250 N aqueous silver nitrate solution-   Detection electrodes:

Composite glass electrode C-172

Silver electrode M-214

Temperature compensation electrode T-111

-   Determination method: A Teflon (registered trademark) stirrer chip    is placed in a 200-mL beaker, and an appropriate amount of a sample    is placed therein (the mass of the sample is measured by means of a    balance). Pure water is added to the beaker so that the volume of    the liquid in the beaker is about 150 mL, and 60 mass % nitric acid    (about 2 mL) is added thereto. Bromide ion concentration is    determined through precipitation titration by means of the    aforementioned automatic potentiometric titrator.

<Method for Determining By-Produced Carboxylic Acid MixtureConcentration>

Concentration of each of the aforementioned phthalic acid compounds andtrimellitic acid compounds was determined through gas chromatographyunder the following conditions.

-   Model: Agilent 6890N (product of Agilent Technologies)-   Column employed: DB-1 (product of Agilent Technologies)-   Column temperature: 100 to 280° C.-   Detector: Flame ionization detector (FID)

Percent recovery of catalyst components and percent inclusion of aby-produced carboxylic acid mixture were calculated through thefollowing methods.

<Cobalt Ion>

Percent recovery of cobalt ions was determined by calculating the ratio(%) of the amount of cobalt ions contained in a recovered catalystliquid to that of cobalt ions contained in a mother liquor.

<Manganese Ion>

Percent recovery of manganese ions was determined by calculating theratio (%) of the amount of manganese ions contained in the recoveredcatalyst liquid to that of manganese ions contained in the motherliquor.

<Bromide Ion>

Percent recovery of bromide ions was determined by calculating the ratio(%) of the amount of bromide ions contained in the recovered catalystliquid to that of bromide ions contained in the mother liquor.

<By-Produced Carboxylic Acid Mixture>

Percent inclusion of a by-produced carboxylic acid mixture wasdetermined by calculating the ratio (%) of the amount of a phthalic acidcompound or trimellitic acid compound contained in the recoveredcatalyst liquid to that of a phthalic acid compound or trimellitic acidcompound contained in the mother liquor.

Example 1

p-Xylene was subjected to liquid-phase oxidation reaction (reactiontemperature: 200° C., reaction pressure: 1.62 MPa) by use of air inhydrous acetic acid having a water content of 9 mass % in the presenceof cobalt acetate, manganese acetate, and hydrobromic acid, to therebyyield a slurry containing crude terephthalic acid. Subsequently, theslurry was subjected to a crystallization step, followed by pressurerelease and cooling to 100° C. Thereafter, the terephthalic acid wasremoved through solid-liquid separation with a glass filter, to therebyrecover crude terephthalic acid crystals and a mother liquor of about80° C. The mother liquor was found to contain cobalt ions in an amountof 610 mass ppm, manganese ions in an amount of 360 mass ppm, bromideions in an amount of 810 mass ppm, trimellitic acid compounds in anamount of 2,500 mass ppm, phthalic acid compounds in an amount of 2,100mass ppm, and water in an amount of 12 mass %. The bromide ratio of themother liquor was found to be 0.6.

The above-preliminarily treated pyridine-ring-containing chelate resin(90 mL) was charged into a double-tube glass column. Hot water of 80° C.was circulated through a jacket of the pyridine-ring-containing chelateresin column, to thereby maintain the pyridine-ring-containing chelateresin at 80° C.

The aforementioned mother liquor was introduced to thepyridine-ring-containing chelate resin column through the top of thecolumn and was caused to pass downward therethrough at a flow rate of360 g/hour for 50 minutes [adsorption step]. Thereafter, hydrous aceticacid having a water content of 12 mass % was introduced to the columnthrough the top and caused to pass downward therethrough at a flow rateof 360 g/hour for 20 minutes (Q/A=120 g/90 mL=about 1.33) [elution step(A)]. After completion of the elution step (A), hydrous acetic acidhaving a water content of 35 mass % was introduced to the column throughthe bottom and caused to pass upward therethrough at a flow rate of 360g/hour for 90 minutes [elution step (B)]. After completion of theelution step (B), an displacement liquid (hydrous acetic acid having awater content of 12 mass %) was introduced to the column through the topand caused to pass downward therethrough at the same flow rate asdescribed above for 20 minutes [displacement step]. A cycle consistingof the adsorption step, the elution step (A), the elution step (B), andthe displacement step (and return to the adsorption step) was repeatedat 180 minutes/cycle.

Hydrous acetic acid having a water content of about 12 mass % wasrecovered, through distillation, from the residual mother liquorobtained through the adsorption step, from the eluate obtained throughthe elution step (A), and from the displacement liquid employed in thedisplacement step. The thus-recovered hydrous acetic acid was reused ashydrous acetic acid in the liquid-phase oxidation reaction, as hydrousacetic acid in the elution step (A), and as a displacement liquid in thedisplacement step. Also, a catalyst liquid recovered through the elutionstep (B) and containing cobalt ions, manganese ions, and bromide ionswas reused as a catalyst for liquid-phase oxidation reaction.

Table 1 shows data of percent recovery of cobalt ions, manganese ions,and bromide ions, and percent inclusion of phthalic acid compounds andtrimellitic acid compounds, as determined by use of a catalyst liquidrecovered on day 4 of the experiment.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 1 Percent recovery Cobalt ions 88% Manganese ions 68% Bromide ions92% Percent inclusion Trimellitic acid compounds 50% Phthalic acidcompounds  9%

Comparative Example 1

The experiment and measurement were performed in the same manner asemployed in Example 1, except that the elution step (A) was notperformed, and the cycle consisting of the adsorption step, the elutionstep (B), and the displacement step (and return to the adsorption step)was repeated at 160 minutes/cycle. The results are shown in Table 2.

The experiment was further continued. The catalytic activity wasgradually reduced, and the yield of terephthalic acid produced throughliquid-phase oxidation reaction was considerably reduced.

TABLE 2 Percent recovery Cobalt ions 89% Manganese ions 79% Bromide ions93% Percent inclusion Trimellitic acid compounds 77% Phthalic acidcompounds 38%

Example 2

The experiment and measurement were performed in the same manner asemployed in Example 1, except that the elution step (A) was performedfor 60 minutes (Q/A=360 g/90 mL=4), and the cycle consisting of theadsorption step, the elution step (A), the elution step (B), and thedisplacement step (and return to the adsorption step) was repeated at220 minutes/cycle. The results are shown in Table 3.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 3 Percent recovery Cobalt ions 88% Manganese ions 67% Bromide ions91% Percent inclusion Trimellitic acid compounds 34% Phthalic acidcompounds  5%

Example 3

The experiment and measurement were performed in the same manner asemployed in Example 1, except that hydrobromic acid was added to therecovered mother liquor so as to attain a bromide ratio of 0.9, and thenthe mother liquor was subjected to the adsorption step. The results areshown in Table 4.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 4 Percent recovery Cobalt ions 96% Manganese ions 72% Bromide ions92% Percent inclusion Trimellitic acid compounds 43% Phthalic acidcompounds  7%

Example 4

The experiment and measurement were performed in the same manner asemployed in Example 1, except that hydrobromic acid was added to therecovered mother liquor so as to attain a bromide ratio of 1.2, and thenthe mother liquor was subjected to the adsorption step. The results areshown in Table 5.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 5 Percent recovery Cobalt ions 98% Manganese ions 77% Bromide ions92% Percent inclusion Trimellitic acid compounds 38% Phthalic acidcompounds  6%

Example 5

The experiment and measurement were performed in the same manner asemployed in Example 1, except that hydrobromic acid was added to therecovered mother liquor so as to attain a bromide ratio of 1.5, and thenthe mother liquor was subjected to the adsorption step. The results areshown in Table 6.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 6 Percent recovery Cobalt ions 99% Manganese ions 81% Bromide ions91% Percent inclusion Trimellitic acid compounds 32% Phthalic acidcompounds  5%

Example 6

The procedure of Example 1 was repeated. However, in Example 6, apyridine-ring-containing chelate resin “REILLEX (registered trademark)425 Polymer” (trade name, product of Reilly) (90 mL) was used; themother liquor was fed to the adsorption step after the bromide ratio hadbeen adjusted to 2.5 through addition of hydrobromic acid; and a cycle[adsorption step (120 min), elution step (A) (20 min, Q/A=120 g/90mL=about 1.33), elution step (B) (80 min), and displacement step (20min)] was repeated at 240 minutes/cycle. Also, hydrous acetic acid(water content: 8 mass %, bromide ion level: 10 mass ppm) recovered fromthe bottom of a distillation column upon removal of water (throughevaporation) at 125° C. (ambient pressure) from an eluate obtainedthrough the elution step (A) was employed as the hydrous acetic acid inthe elution step (A) and as the displacement liquid in the displacementstep. Table 7 shows data of percent recovery of cobalt ions, manganeseions, and bromide ions, and percent inclusion of phthalic acid compoundsand trimellitic acid compounds, as determined by use of a catalystliquid recovered on day 180 of the experiment.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 7 Percent recovery Cobalt ions 99.9%   Manganese ions 90% Bromideions 96% Percent inclusion Trimellitic acid compounds  3% Phthalic acidcompounds  0%

Example 7

The experiment was performed in the same manner as employed in Example6, except that the recovered mother liquor was heated to 90° C., andthen subjected to the adsorption step. Table 8 shows data of percentrecovery of cobalt ions, manganese ions, and bromide ions, and percentinclusion of phthalic acid compounds and trimellitic acid compounds, asdetermined by use of a catalyst liquid recovered on day 10 of theexperiment.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 8 Percent recovery Cobalt ions 99.9%   Manganese ions 98% Bromideions 99% Percent inclusion Trimellitic acid compounds 1.8%  Phthalicacid compounds  0%

Example 8

The experiment was performed in the same manner as employed in Example6, except that the recovered mother liquor was cooled to 40° C. so as toremove a precipitated solid content, and then subjected to theadsorption step, and that the pyridine-ring-containing chelate resin wasemployed at 40° C. Table 9 shows data of percent recovery of cobaltions, manganese ions, and bromide ions, and percent inclusion ofphthalic acid compounds and trimellitic acid compounds, as determined byuse of a catalyst liquid recovered on day 7 of the experiment.

The experiment was further continued. However, liquid-phase oxidationreaction was effectively performed without reducing catalytic activity.

TABLE 9 Percent recovery Cobalt ions 84% Manganese ions 63% Bromide ions81% Percent inclusion Trimellitic acid compounds  5% Phthalic acidcompounds  0%

As is clear from the data obtained in Examples 1 to 8, according to themethod of the present invention, percent inclusion of trimellitic acidcompounds is reduced to 50% or less, and percent inclusion of phthalicacid compounds is reduced to 10% or less. In addition, under certainconditions, the percent inclusion is reduced to 5% or less. Particularlywhen the time of the elution step (A) (i.e., Q/A) is regulated asdescribed in Example 2, or when the bromide ratio of a mother liquor ismodified before the adsorption step (Examples 3 to 5), percent inclusionof trimellitic acid compounds and phthalic acid compounds is furtherreduced, as compared with the case of Example 1. In contrast, accordingto the method of Comparative Example 1, which does not include theelution step (A) and thus is similar to a conventional productionmethod, percent inclusion of trimellitic acid compounds is 77%, andpercent inclusion of phthalic acid compounds is increased to 38%. Thatis, this method encounters difficulty in continuously performingliquid-phase oxidation reaction over a long period of time while reusingrecovered catalyst components. In Examples 6 to 8, the bromide ratiobefore the adsorption step was adjusted to 2.5, and the bromide ionlevel of the hydrous acetic acid employed in the elution step (A) and inthe displacement step was adjusted to 10 mass ppm. In these Examples,percent inclusion of trimellitic acid compounds can be successfullyreduced to a very low level, and percent inclusion of phthalic acidcompounds can be reduced to substantially zero. Thus, remarkable effectscan be attained. When the temperature of a mother liquor is increased to90° C. (Example 7), percent recovery of heavy metal ions is increased,and percent inclusion of trimellitic acid compounds and phthalic acidcompounds is reduced.

A difference in percent inclusion of a by-produced carboxylic acidmixture by some tens % greatly affects production of terephthalic acid.Thus, the present invention realizes economically advantageous andlong-term reliable production of terephthalic acid.

INDUSTRIAL APPLICABILITY

Terephthalic acid produced through the method of the present inventioncan be employed as, for example, a starting material for polyethyleneterephthalate.

1. A method for producing terephthalic acid comprising: subjecting ap-phenylene compound to a liquid-phase oxidation reaction by the use ofa molecular-oxygen-containing gas in the presence of a catalyst at leastcontaining a heavy metal compound and a bromine compound, and hydrousacetic acid having a water content of 1 to 15 mass %, to thereby yield aslurry; regulating the temperature of the slurry to 35 to 140° C., tothereby cause terephthalic acid to precipitate; removing theterephthalic acid through solid-liquid separation, to thereby recover amother liquor; and recovering the catalyst from the mother liquorthrough a series of operations (1) to (4) for reusing at least a portionof the catalyst in the liquid-phase oxidation reaction: (1) anadsorption operation including regulating the amount by mole of bromideions in the mother liquor/total amount by mole of heavy metal ions inthe mother liquor ratio to 0.6 to 3, and then exposing the mother liquorto a pyridine-ring-containing chelate resin which has been heated to 35to 140° C., so that the resin adsorbs catalyst-derived heavy metal ionsand bromide ions, and also adsorbs a by-produced carboxylic acid mixturewhich has been by-produced through the liquid-phase oxidation reaction,(2) an elution operation (A) of exposing hydrous acetic acid having awater content of 1 to 15 mass % to the pyridine-ring-containing chelateresin which has undergone the adsorption operation, thereby yielding aneluate containing the by-produced carboxylic acid mixture, (3) anelution operation (B) of exposing water or hydrous acetic acid having awater content of 20 mass % or more to the pyridine-ring-containingchelate resin which has undergone the elution operation (A), therebyyielding an eluate containing catalyst-derived heavy metal ions andbromide ions, and (4) a displacement operation of exposing hydrousacetic acid having a water content of 1 to 15 mass % to thepyridine-ring-containing chelate resin which has undergone the elutionoperation (B), serving as a displacement liquid, thereby regeneratingthe resin.
 2. The method for producing terephthalic acid as described inclaim 1, wherein hydrous acetic acid is recovered from the mother liquorwhich has undergone the adsorption operation, from the eluate obtainedthrough the elution operation (A), and from the displacement liquidemployed in the displacement operation, and the recovered hydrous aceticacid is reused in the liquid-phase oxidation reaction as at least aportion of hydrous acetic acid having a water content of 1 to 15 mass %.3. The method for producing terephthalic acid as described in claim 1,wherein hydrous acetic acid is recovered from the mother liquor whichhas undergone the adsorption operation, from the eluate obtained throughthe elution operation (A), and from the displacement liquid employed inthe displacement operation, and the recovered hydrous acetic acid isreused in the elution operation (A) as at least a portion of hydrousacetic acid having a water content of 1 to 15 mass %.
 4. The method forproducing terephthalic acid as described in claim 1, wherein hydrousacetic acid is recovered from the mother liquor which has undergone theadsorption operation, from the eluate obtained through the elutionoperation (A), and from the displacement liquid employed in thedisplacement operation, and the recovered hydrous acetic acid is reusedin the displacement operation as a displacement liquid.
 5. The methodfor producing terephthalic acid as described in claim 1, wherein theeluate obtained through the elution operation (B) is returned to theliquid-phase oxidation reaction, and reused as at least a portion of thecatalyst.
 6. The method for producing terephthalic acid as described inclaim 1, wherein a regenerated pyridine-ring-containing chelate resinwhich has been obtained through the displacement operation is reused inthe adsorption operation as the pyridine-ring-containing chelate resin.7. The method for producing terephthalic acid as described in claim 1,wherein the hydrous acetic acid having a water content of 1 to 15 mass %and employed in the elution operation (A) contains bromide ions in anamount of 1 to 1,000 mass ppm.
 8. The method for producing terephthalicacid as described in claim 1, wherein the hydrous acetic acid having awater content of 1 to 15 mass % and employed in the displacementoperation as a displacement liquid contains bromide ions in an amount of1 to 1,000 mass ppm.
 9. The method for producing terephthalic acid asdescribed in claim 1, wherein, in the adsorption operation, the amountby mole of bromide ions in the mother liquor/total amount by mole ofheavy metal ions in the mother liquor ratio is regulated to 1.6 to 2.5.10. The method for producing terephthalic acid as described in claim 1,wherein when Q represents the total amount (g) of hydrous acetic acidhaving a water content of 1 to 15 mass % supplied in the elutionoperation (A), and V represents the volume (mL) of apyridine-ring-containing chelate resin layer, the ratio Q/V is 0.5 to10.